Travelling wave tubes having multiple
slow wave structures



June 28, 1966 B. MINAKOVIC 3,258,640

TRAVELLING WAVE TUB ES HAVING MULTIPLE SLOW WAVE STRUCTURES Filed March8, 1961 3 Sheets-Sheet l I nventor B .I Iin kovic By Attorney June 28,1966 B. MlNAKOVlC TRAVELLING WAVE TUBES HAVING MULTIPLE SLOW WAVESTRUCTURES S Sheets-Sheet 2 Filed March 8, 1961 FIG.4.

FIGS.

FIG. 8.

Inventor B .Minakovi 0 y W A Horn e y June 28, 1966 B. MlNAKOVlC ,258,

TRAVELLING WAVE TUBES HAVING MULTIPLE SLOW WAVE STRUCTURES Filed March8, 1961 3 Sheets-Sheet 5 F|G.6. r

rorauicrafi Q END S-w E Q Q E E Q 5 g L I t HEBQJ E/VQ FIG].

29 II v Inventor B .l-iinakovic Attorney United States Patent 3,258,640TRAVELLING WAVE TUBES HAVING MULTIPLE SLOW WAVE STRUCTURES BorivojeMinakovic, London, England, assignor to International Standard ElectricCorporation, New York,

Filed Mar. 8, 1961, Ser. No. 94,269 Claims priority, application GreatBritain, Mar. 24, 1960, 10,408/ 60 Claims. (Cl. 315-3.6)

The present invention relates to travelling wave tubes and isparticularly concerned with the construction of slow wave structurestherefor. The term travelling wave tube includes backward waveoscillator or amplifier tubes in which the energy of wave propagationalong the slow wave structure is oppositely directed to the electronstream.

In the most commonly used travelling wave tubes an electron beam isprojected from an electron gun along the axis of a wire helix to anelectron collector electrode. The dimensions of the helix are chosen toprovide slow wave propagation along the axis of the helix with a phasevelocity approximately the same as that of the electrons of the beam.These dimensions, therefore, depend, inter alia, upon the desiredfrequency of operation of the travelling Wave tube and also upon theaccelerating voltage applied to the electrons of the beam. Thus, intubes for operation in the 4-6 krnc./s. band with a beam voltage of3,000 volts the helix is typically made of wire 0.010" diameter, woundat 30 turns per inch with the helix having a mean diameter of 0.090".Such a helix is conveniently and conventionally supported between threeparallel dielectric rods extending between the electron gun and electroncollector electrode. If it were desired to operate a travelling wavetube at a comparatively low beam accelerating voltage-say, 100volts-even at much lower frequencies, the helix would have to be woundof very fine wire at many turns per unit axial length. Diflicultieswould then arise in supporting the helix and avoiding distortion to itsgeometry; the conventional three rod support becomes impracticable.

At very high frequencies, say in the millimetre wave range, irrespectiveof beam voltage considerations, the helix slow wave structure becomesextremely difficult to support. For this and other reasons recourse ismade to devices such as ladder types of slow wave structure, examples'ofwhich will be discussed below.

In the present invention a slow wave structure is used which has anarrangement surrounding the electron beam of a set of slow wavestructures whose fields couple to one another and to the electron beamin such a manner as to provide along the electron beam path a means forthe propagation of electro-magnetic Waves in a slow mode in which theelectric vector is predominantly longitudinal. Each of the structures,of which there are at least three, is of the same class of slow wavestructure. Thus, the structures may all be helices or they may all beladder type structures of similar form. The structures are arrangedparallel to one another, side-by-side, symmetrically around the electronbeam axis and should not overlap in planes transverse to that axis-Le.in the case of helices, for example, the turns of one helix shouldneither touch nor overlap those of its neighbour. I

In embodiments of the invention using a set of helices around theelectron beam to provide the composite slow wave structure it becomespracticable to wind the helices of very fine wire on dielectric rodswhich, in the assembled tube, remain as the supports for the respectivehelices, thus overcoming the difficulties mentioned above in connectionwith low voltage travelling wave tubes. For use at very high frequenciesit is possible to provide a set of several ladder type structuresarranged parallel to one Patented June 28, 1966 another and to flood thewhole assembly with an electron beam. In the present invention theladder type structures can be arranged around the beam so thatinterception of the beam electrons is largely avoided.

Embodiments of this invention will be described, with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of a travelling wave tubeaccording to the invention employing a set of helices for the slow wavestructure;

FIG. 2 is a cross section through the plane IIII of FIG. 1;

FIGS. 3 and 4 illustrate the general arrangement of the individualhelices in embodiments such as that of FIGS. 1 and 2;

FIG. 5 illustrates the electric field configuration produced in thearrangement of FIGS. 3 and 4;

FIG. 6 illustrates an alternative form of h.f. feed for the slow wavestructure; and

FIGS. 7 to 9 relate to ladder type slow wave structures, FIGS. 7 and 9showing respective arrangements according to the invention of structuresderived from the basic ladder structure of FIG. 8.

In the embodiments of FIGS. 1 and 2 an electron gun 1 having a cathode 2focusing electrode 3 and anode 4 is mounted within an envelope portion5, from which projects an elongated envelope portion 6 of smaller diameter surrounding the electron beam path and the composite slow wavestructure and which at its far end carries an electron collectorelectrode 7. The composite slow wave structure comprises a set ofhelices each wound upon a dielectric rod 8 the rods being parallel tothe electron beam axis and symmetrically arranged about that axis asshown in FIG. 2. The dielectric rods are supported at their ends in apair of metal sleeves 9. Input and output couplings for the compositeslow wave structure are provided by externally contra-wound helicalcouplers illustrated at 10.

The general arrangement of the composite slow wave structure isillustrated in FIGS. 3 and 4. In FIG. 3, eight similar helices 11 areshown, each wound upon its own support rod 8. In any plane transversethe electron beam axis the centres of the rods lie upon a circle whosecentre is the electron beam axis and are spaced from one another by adistance d as indicated in FIG. 4. The electron beam indicated in FIG. 3at 12 passes close to the helices so that the beam electrons interactwith the external fields of the helices. A simple transmission linetheory shows that irrespective of whether all the helices are wound inthe same direction, or, in the case of an even number, alternate onesare contrawound, two modes of slow wave propagation are possible. Thefaster of these modes has a predominantly longitudinal electric field asillustrated in FIG. 5. The lines of electric force 13 flow from node tonode along the length of each helix so that the electric vector ispredominantly longitudinal. In the other mode of propagation lines ofelectric force tend to be directed transversely from on helix to itsadjacent neighbour so that any electric field near the axis ispredominantly transverse. In the present invention the mode illustratedin FIG. 5the in-phase modeis utilised. The propagation constant of thecomposite slow wave structure may be expressed in terms of thepropagation constants of th individual helices, together with termsinvolving the distance d. For maximum band Width and freedom fromunwanted Hartree harmonics we prefer to dimension the individual helicesso that ya for each helix is approximately 1.5, being the radial phaseconstant of propagation and a the radius of the helix. These terms aretaken from the known Hartree equation, a discussion and derivation ofwhich is found on pages 72, 73 and 76 of the text entitled TravellingWave Tubes by J. R. Pierce, 1950 edition. For a six helix structure witheach helix dimensioned so that 'ya=1.5 the simple transmission linetheory predicts that the interaction impedance of the composite slowwave structure with the electron beam should be about 0.45 of the singlehelix interaction impedance. The effect of the dielectric of the supportrods on which the individual helices are wound is to reduce theinteraction impedanc still further. On the other hand, since thein-phase mode is the faster of the two slow modes of propagation, theinteraction impedance for the composite structure will be higher thanthat given by the simple theory, so that a compensating effect isobtained. The six helix structure is more dispersive than a singlehelix, and this is, in general, true for any member of slow wavestructures whether of the helix class or otherwise. The dispersion canbe increased when using the helix class by contrawinding alternatehelices or adjacent groups of helices, the symmetry of the arrangementstill being preserved. The dispersion may also b varied by choice of thespacing of.

If a small number of slow wave structures is used to form the compositeslow wave structure, interaction with the electron beam will berelatively inefficient, while if a large number is used the field on theaxis tends to fall off because of the greater distance of the individualslow wave structures from the common axis, hence the use of a largenumber of slow wave structures is suitable for a hollow electron beam.For a solid electron beam, considerations of dispersion and interactionlead to a choice of six helices as being the optimum number. If thespacing d between individual helices approaches th diameter of a helixthe interaction with the electron beam becomes very inefficient. As d isdecreased the dispersion of the composite structure increases withconsequent loss of band width.

In a practical embodiment of the invention a six helix structure wasused, all helices being wound in the same direction. The helix was woundwith 0.002 inch diameter tungsten wire at 250 turns per inch on ceramicrods each of diameter 0.075 inch. The length of each helix was 4 /2inches with a beam 0.083 inch in diameter carrying a current of 2 ma.with a beam voltage centered on 100 v. The beam diameter was chosen asin conventional single helix travelling wave tubes to occupy about 0.8of the available spacing. A maximum gain of 20 db was obtained at 760mc./s. and 118 volts on the beam. The amplifier was voltage-tuned byvarying the beam voltage over the range 86-125 volts, the average gainin the frequency range 1020-710 mc./s. being about db.

In the arrangement illustrated in FIG. 1, coupling to the slow wavestructure is indicated as by means of contra-wound helix couplers 10. IfP be the pitch of the individual helices of the slow wave structure andF be the pitch of the outer coupling helix then, approximately where Dis the mean diameter of coupling helix and S is the mean distancebetween coupling helix and inner helix.

As in the case of a conventional helical coupler the power istransferred to the structure by a spatial heating process. The couplinglength of the outer helix can be found very approximately from the usualhelical coupler theory. This value, however, will be shorter thanactually required because the spacing between inner helices tends toreduce the coupling to the outer helix.

In the alternative feeder arrangement illustrated in FIG. 6 waveguidecoupling is used. The individual helices are closely surrounded by aglass envelope 19, which projects in an analogous manner to theconventional travelling wave tube through a waveguide 20. The ends ofthe helices terminate flush with the inner wall of the waveguide. Ametal drift tube 21, held in between the helices of the slow wavestructure and surrounding the electron beam, projects transverselyacross the waveguide and is joined to a choke sleeve 22, whichco-operates with a flange 23, projecting from the waveguide wall to forma conventional waveguide choke for passage of the envelope 19.

Besides the two arrangements described above for coupling power into orout of a slow wave structure according to the invention, various othermethods may be adopted. Thus, in the case of an even number of helices,alternate ones of which are contrawound and which are closely spaced soas to provide strong coupling between adjacent helices, it is sufficientto couple a waveguide or Coaxial feeder to only one of them, usingwaveguide probe or other forms of coupler such as conventionally used intravelling wave tubes. It is not necessary, in any case, that there bedirect D.C. coupling between the several helices unless it be for beamfocusing reasons.

Turning now to the use of other types of slow wave structure, FIG. 7illustrates an arrangement of four juxtaposed ladder type slow wavestructures, together forming a composite slow wave structure for use inhigh frequency embodiments of the invention. Each component slow wavestructure of FIG. 7 can be regarded as derived from the ladder structureof FIG. 8 which is made up of a pair of side plates 24, between whichstretches a transverse grating 25 of parallel resonators, here shown asslots, but which could be bars, inclined to the normal between the sideplates 24 so as to produce an assy-mmetry in the electric fielddistribution and enable it to propagate over a finite band offrequencies. In FIG. 7 the slotted grating 26 corresponds to the grating25 of FIG. 8 and the vanes 27, which are now turned each outwardsthrough an angle of 45, correspond to the upper parts of the side plates24. The lower skirts of the side plates 24 are replaced by the adjacentslotted gratings 28 and 219. Similarly the adjacent slow wave structurehas the grating 28 corresponding to the grating 25, the vanes 27corresponding to the upper portions of the skirts 24 and the adjacentgratings 26 and 30 corresponding to the lower skirts of the side walls24.

FIG. 9 shows diagrammatically an end view of an arrangement of six slowwave structures of the ladder type assembled together in an analogousmanner to the four structures of FIG. 7 and comprises webs 27 andsimilar gratings 31. The gratings 31 can take other forms than thoseillustrated in FIG. 8 such for example, as an intersecting structure ofconductors forming a mesh such as disclosed in U.S. Patent No.3,090,886, issued May 21, 1963, and assigned to the same assignee as theinstant application.

In embodiments using the arrangements of FIGS. 7 and 9, the electronbeam is projected along the axis of the composite slow wave structure,as in the case of the multiple helix embodiments. Coupling to input andoutput feeders may be effected in various ways known to those skilled inthe art; one method of coupling would be to mount the composite slowwave structure between input and output waveguides normal to the axis ofthe electron beam, as in many conventional travelling wave tubes, and tohave extensions of the individual grating projecting into the respectivewaveguides through slots cut in the waveguide walls.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What I claim is:

1. A travelling wave tube including an envelope means for projecting anelectron beam along a given axis of said envelope and at least threeslow wave structures of the same form positioned radially andsymmetrically about said beam with the axes of propagation of saidstructures parallel to said given axis and the electric field having apredominantly longitudinal mode, each said structure beingelectromagnetically coupled to and spaced apart from the electron beamand the adjacent structure, and signal coupling means around saidenvelope adjacent and spaced from the ends of said slow wave structurefor electromagnetically coupling power into and out of respective saidends.

2. A travelling wave tube according to claim 1 in which the slow wavestructures are helices whose pitch circles are spaced apart from oneanother and from said beams.

3. A travelling wave tube according to claim 1 in which each slow wavestructure is formed by a uni-planar grating of parallel conductorsarranged in a mesh formation between a pair of side walls.

4. A travelling wave tube according to claim 2 wherein said helices areeach wound in the same direction.

5. A travelling wave tube according to claim 2 wherein adjacent saidhelices are wound alternately in opposite directions.

6. A travelling wave tube including an envelope, an electron gun beamforming structure, a slow wave structure and an electron collectorelectrode aligned along a given axis of said envelope, in which the slowwave structure is formed by a set of at least three similar helices eachWound upon a dielectric support rod, and signal coupling means aroundsaid envelope adjacent and spaced from the ends of said helices forelectromagnetically coupling power into and out of respective said endsthe travelling wave tube further including means mounting the helicesand rods symmetrically surrounding and parallel to the given axis andbeam in an electromagnetically coupled spaced relationship with theelectron beam and with one another, and the electric field within saidtube having a predominantly longitudinal mode.

7. A travelling wave tube according to claim 6 in which the set containssix helices.

8. A travelling wave tube according to claim 7 in which the ends of eachrod are mounted in a metal sleeve and each helix is joined at itsrespective ends to the adjacent sleeve.

9. A travelling wave tube according to claim 7 wherein said signalcoupling means includes a Waveguide feeder and means for coupling theslow wave structure thereto, said coupling means for said waveguidefeeder comprising a drift tube having one end inserted between thehelices of the slow wave structure and surrounding the electron beam,said drift tube extending transversely through said waveguide feeder,and a choke at the other end slee ve mounted on the far end of the drifttube continuous with the opposite waveguide wall.

10. Apparatus including a travelling Wave tube according to claim 7wherein said signal coupling means includes a contrawound helix couplersurrounding the travelling wave tube about one end of the slow wavestructure.

References Cited by the Examiner UNITED STATES PATENTS 2,679,019 5/ 1954Lindenblad 315- 2,789,247 4/1957 Jonker 315--39.3 X 2,801,361 7/1957Pierce 3153.6 2,831,142 4/1958 Kazan 3153.6

DAVID J. GALVIN, Primary Examiner.

GEORGE WESTBY, ARTHUR GAUSS, V. LAFRAN- CHI, G. R. OFELT, Examiners. i

1. A TRAVELLING WAVE TUBE INCLUDING AN ENVELOPE MEANS FOR PROJECTING ANELECTRON BEAM ALONG A GIVEN AXIS OF SAID ENVELOPE AND AT LEAST THREESLOW WAVE STRUCTURES OF THE SAME FORM POSITIONED RADIALLY ANDSYMMETRIALLY ABOUT SAID BEAM WITH THE AXES OF PROPAGATION OF SAIDSTRUCTURES PARALLEL TO SAID GIVEN AXIS AND THE ELECTRIC FIELD HAVING AREDEMINANTLY LONGITUDINAL MODE, EACH SAID STRUCTURE BEINGELECTROMAGNETICALLY COUPLED TO AND SPACED APART FROM THE ELECTRON BEAMAND THE ADJACENT STRUCTURE, AND SIGNAL COUPLING MEANS AROUND SAIDENVELOPE ADJACENT AND SPACED FROM THE ENDS OF SAID SLOW WAVE STRUCTUREFOR ELECTROMAGNETICALLY COUPLING POWER INTO AND OUT OF RESPECTIVE SAIDENDS.